Stroke, a neurological disorder with a high incidence in the elderly, is a leading cause of long-term disability in adults worldwide. Stroke is also an enormous public health problem and a serious public financial burden in the United States. Chronic stroke is identified as the period beyond 3-6 months after stroke onset. Currently, no pharmaceutical treatment is available for chronic stroke victims. Stem cell factor (SCF) and granulocyte- colony stimulating factor (G-CSF) are hematopoietic growth factors (HGFs) that regulate blood cell production and mobilize bone marrow stem cells (BMSCs) into the blood. Our recent publications have shown that the receptors for SCF and G-CSF are expressed by neurons, that both SCF and G-CSF can pass through the blood-brain barrier, and, more importantly, that SCF in combination with G-CSF (SCFCSF) shows therapeutic benefits for chronic stroke in a rat model of stroke. However, it remains entirely unknown how SCFCSF repairs a permanently damaged brain during chronic stroke. The objective of this project is to address this unanswered question. Supported by considerable preliminary data, the central hypothesis is that neuronal network remodeling contributes to SCFCSF-induced functional recovery in chronic stroke and that the neuronal network remodeling is accomplished by the direct effects of SCFCSF on neurons and the indirect effects via BMSCs. Using neuronal and BMSC cultures together with stroke models in aged spontaneously hypertensive rats and in aged wild type or transgenic C57 BL mice expressing yellow or cyan fluorescent proteins only in neurons (Thy-1-YFP, Thy-1-CFP), and bone marrow transplantation to track BMSCs, this hypothesis will be tested by pursuing two specific aims. Aim #1 is to determine whether SCF CSF-induced functional benefits in chronic stroke are dependent upon MEK/ERK and PI3k/Akt pathway- mediated neuronal network remodeling. Western blots, cell signaling blockade, RNA interference, 2-photon live imaging and neurological deficit examinations will be used. Aim #2 is to identify the role of the S100A8/A9 chemical gradient established by SCFCSF in homing BMSCs to the brain and to determine the contribution of the BMSCs to SCFCSF-induced functional recovery from chronic stroke. BMSC migration assays, S100A8/A9 gene silencing by RNA interference, and sensorimotor functional evaluation will be utilized. The proposed research is significant because it will advance and extend understanding of how HGFs repair the brain after permanent damage by stroke. Additionally, it will significantly contribute to the development of a unique therapeutic strategy for treatment of chronic stroke. This contribution is in keeping with the NIH mission to reduce national and personal burdens caused by illness and disability. PUBLIC HEALTH RELEVANCE: The proposed study focuses on a highly important, but under-investigated topic: a new therapeutic strategy to treat chronic stroke in the aged population. This research is directly relevant to public health. The findings can provide fresh insights into the contribution of hematopoietic growth factors in brain repair. Additionally, this knowledge may also lead to the development of new treatments for other neurological disorders and neurodegenerative diseases.